At present, 3rd Generation Partnership Project (3GPP) is reviewing the specifications of Long Term Evolution (LTE) systems and LTE-Advanced systems. For LTE, the specifications of LTE Releases 8 to 12 have been defined. In Japan, various telecommunications carriers are providing services on the basis of Release 8.
Further, LTE-Advanced systems (release 10 and later), which are developed forms of LTE systems, have currently been reviewed. The specifications have been defined up to Release 12, and Release 13 is being reviewed. In Korea, services were started in June 2013. In Japan, one telecommunications carrier started a service in June 2014, and another telecommunications carrier also started a service in March 2015.
In LTE Release 10, i.e., LTE-Advanced systems, a configuration described below, for example, may be used. An LTE-Advance system includes a base station (or a base station apparatus; both of which will hereinafter be referred to as a “base station”) called an “evolved Node B (eNB)” and a communication terminal (or a terminal; both of which will hereinafter be referred to as a “communication terminal”) called a “User Equipment (UE)”. Further, the base station serves as a transmitting apparatus (a transmitter or a transmitting station) that performs downlink transmission to the communication terminal and also serves as a receiving apparatus (a receiver or a receiving station) that receives uplink signals from the terminal. Similarly, the communication terminal serves as a receiving apparatus (a receiver or a receiving station) that receives downlink transmission from the base station and also serves as a transmitting apparatus (a transmitter or a transmitting station) that performs uplink transmission to the base station. Further, the LTE-Advanced system includes a Mobility Management Entity (MME) configured with a controlling apparatus that connects to the Internet called a core network. Further, the LTE-Advanced system includes a Serving Gate Way (S-GW) configured with a server used for transferred data such as user data. Further, the LTE-Advanced system includes S1 serving as an interface between the MME/S-GW and the eNB, as well as X2 serving as an interface between eNBs. In this situation, S1 and X2 are each an interface using Transmission Control Protocol/Internet Protocol (TCP/IP).
Further, the base station has one cell, which is a communication area, and performs communication with any of the communication terminals contained in the cell. Also, as a result of communication performed between base stations, communication terminals contained in mutually the same cell or mutually different cells are able to communicate with one another. In this regard, because each base station has only one band, the terms “base station”, “cell”, and “band” may be treated as having the same meaning in the explanations below.
Further, other examples of configurations of LTE-Advanced systems include the following: An LTE-Advanced system includes an HeNB (Home eNB) of which the cell is smaller than regular cells and which may be installed indoor (e.g., in a home or an office) and an HeNB GW serving as a server for the HeNB. Further, the LTE-Advanced system may include Si serving as an interface between the MME/S-GW and the eNB and HeNB. Further, the LTE-Advanced system may include X2 serving as an interface between eNBs, between HeNBs, and among the eNB, an X2-GW, and the HeNB. Further, for the communication between a base station and a communication terminal, a relay apparatus (a relay node) may be used to realize a relay transfer.
For these LTE-Advanced systems, a technique called Carrier Aggregation (CA) has been proposed. In LTE systems, it is possible to set an uplink/downlink bandwidth (or a system bandwidth) to 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. The band set in this situation is defined as a component carrier. The reason why the plurality of bandwidths are set is that the configuration is based on the premise that bandwidths allocated to conventional systems such as Global System for Mobile communication (GSM [registered trademark]) systems and Wideband Code Division Multiple Access (W-CDMA) systems will be used without any modification applied thereto.
However, it is desirable to configure LTE systems so as to be able to transfer data at higher speeds than in the conventional GSM and W-CDMA systems. For this reason, it is desirable to configure LTE systems to have a wider bandwidth than the conventional systems. Generally speaking, bands used in radio communication systems vary in accordance with circumstances in each country. Further, because the countries in Europe border one another by land, the frequency bands being used are adjusted among countries in consideration of interference. As a result, the bandwidths that are usable in radio communication systems in each country are small in number and chopped in small pieces. To cope with this situation, for the purpose of realizing wider bands in LTE systems, a method has been introduced to obtain wider bands by integrating the small and chopped-up bands together. The method for obtaining wider bands by integrating the small and chopped-up bands together is the CA process. In other words, the CA is a technique by which communication is performed between at least one transmitting apparatus and at least one receiving apparatus by using a plurality of frequency bands at the same time and is a technique by which communication is performed between one transmitting apparatus and at least one receiving apparatus by using a plurality of communication frequency bands at the same time. As long as these conditions are satisfied, possible configurations of the radio communication system of the present disclosure are not limited to those using the CA.
When a CA process is performed, a cell using a main cell is set. This cell is called a primary cell. The primary cell may be referred to as the first cell, the first band, a main band, or a main cell. In the following sections, a primary cell will be referred to as a “PCell”.
In a CA process, a cell is added to or integrated with the PCell. The additional cell is called a secondary cell. The secondary cell may be referred to as the second cell, a secondary band, an extended band, or a subband. In the following sections, a secondary cell will be referred to as an “SCell”.
These cells are obtained by dividing a band allocated to a system (e.g., W-CDMA or LTE) into sections on the basis of the frequency bandwidth structuring the system (a system bandwidth). It is possible to multiplex users in each band, i.e., to realize multiple access. Further, by scheduling radio resources of data channels that use each of the bands and allocating the radio resources to one or more terminals, it is possible to realize user multiplex. These cells can structure one system. In other words, these cells are different from using blocks (sets or clusters) that are obtained by putting multiple subcarriers together to form allocation units of radio resources for the purpose of realizing user multiplex in an Orthogonal Frequency-Division Multiple Access (OFDMA) scheme.
In CA processes defined in LTE Releases 10 to 12, it is possible to set seven SCells at maximum. In other words, it is possible to realize a CA process by using eight Component Carriers (CCs) at maximum, including a PCell. That is to say, the CA process is a technique used for integrating the PCell with at least one SCell. At present, it is considered to set the maximum number of SCells to 32. Further, CA processes may be classified depending on whether the frequencies of the PCell and the SCell are continuous or not continuous, and whether the frequencies are included in mutually the same frequency band or not. Further, CA processes may be classified depending on whether control information used for data communication that uses an SCell is transferred by the SCell or transferred by a PCell or another SCell. In this situation, the data communication using an SCell uses a Physical Downlink Shared Channel (PDSCH), which is a downlink radio shared channel. Further, the control information for the data communication using an SCell is transmitted by using a Physical Downlink Control Channel (PDCCH), which is a downlink radio control channel.
In addition, as a method for further increasing the communication capacity, it has been put into practice to reduce the number of communication terminals contained in each cell and to increase the communication speed of each communication terminal, by reducing the size of each cell to a smaller area. Such cells having a smaller area are called micro cells, pico cells, femtocells, or small cells. Further, a configuration has been introduced in which, when a CA process is introduced, the PCell is arranged to be a macro cell (a large-area cell), whereas a cell having a smaller area as described above is used as the SCell.
The configuration of the CA process in which the PCell is configured with a macro cell, while the SCell is configured with a cell which has a smaller area and at least a part of which overlaps the PCell may be referred to as an umbrella cell configuration or a hierarchical cell configuration. Examples of methods for realizing the umbrella cell configuration include the following: One method is to connect the PCell and the SCell to each other by using an X2 interface, which is an inter-base-station interface and to transfer user data between the PCell and the SCell. Another method is to perform a signal processing process in the PCell, to convert either a baseband signal or a radio signal into an optical signal, and to connect the PCell and the SCell to each other. Yet another method is to connect the PCell and the SCell to each other with regular radio communication. It is possible to select and use any of these methods depending on the purpose of use and application.
Further, for cellular systems, the frequency bands to be used are determined by law, in consideration of circumstances in each country, on the basis of international allocations of frequencies. Examples of cellular systems include Wideband Code Division Multiple Access (W-CDMA), LTE, LTE-Advanced, and Worldwide Interoperability for Microwave Access (WiMAX) (registered trademark). Further, the frequency bands thereof are assigned to telecommunications carriers by using methods such as an auction among the telecommunications carriers. In other words, as a result of designating a frequency band to be used and giving a license to each telecommunications carrier, each of the telecommunications carriers is permitted to use the designated frequency band for use. The frequency bands of which the use is permitted in this manner are called “licensed bands”.
In contrast, there is another system in which it is possible to perform communication without having a license, by performing the communication with transmission power equal to or lower than a maximum transmission power level determined by law. This system is called a specified low power system. Further, there are also frequency bands in which frequencies can freely be used without a license, as long as the transmission power is equal to or lower than a level determined by law, such as an Industry Science Medical (ISM) band or the 5 GHz band. These frequency bands that are usable without a license are called “unlicensed bands”. Examples of systems that use unlicensed bands include a Wireless Fidelity (Wi-Fi) (The institute of Electrical and Electronics Engineers, Inc. [IEEE] 802.11a).
Unlicensed bands are freely used by a large number of systems such as a plurality of Wi-Fi systems. Accordingly, on the basis of the philosophy of the Radio Act which states that communication of other systems shall not be hindered, Wi-Fi uses, for example, Carrier Sense Multiple Access/Collison Avoidance (CSMA/CA). CSMA/CA is a scheme by which, before data or the like is transmitted on a certain frequency, it is checked to see whether or not the frequency is being used by any other system. With this arrangement, it is possible to prevent the hindrance on other communication activities. Further, in an unlicensed band, no specific system is able to use a frequency in an exclusive manner.
In this regard, as mentioned above, the CA process is introduced to LTE-Advanced for the purpose of increasing the communication volume. Further, by increasing the frequency bands in use and by making the frequency higher, the goal of increasing the communication volume has been addressed. An example of increasing the frequency bands in use is realized by adding the 3.5 GHz band to the 1.7 GHz used by mobile phones and the like. However, we are facing a situation where only increasing the frequency bands in use is not able to address the goal of increasing the communication volume. In other words, because the frequency resources are finite, a problem is arising where usable frequencies may be exhausted.
To cope with this situation, conventional techniques are known by which an unlicensed band and a licensed band are used. For example, according to a conventional technique, on the basis of registered system IDs, a licensed band is used outdoor, whereas an unlicensed band is used indoor. Further, according to another conventional technique, a carrier aggregation process is performed by using a licensed band and an unlicensed band. According to yet another conventional technique, a carrier aggregation process is performed in a licensed band and an unlicensed band while using group IDs, and a synchronization process is performed by using the licensed band.
Examples of techniques related to LTE include a conventional technique by which control is exercised while using numbers or IDs in an LTE network.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-018642
Patent Document 2: Japanese National Publication of International Patent Application No. 2015-505436
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Non Patent Document 1: TS23.003V8.16.0 “3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Numbering, addressing and identification (Release 8)”
Non Patent Document 2: TS23.003V8.16.0 “3rd Generation Partnership Project; Technical Specification Radio Access Network Meeting *65 RP-141664 Edinburgh, Scotland, 9-12 September 2014”
According to the conventional techniques, however, a problem remains as to how the communication terminal is made to recognize the frequency of the unlicensed band as the frequency to be used in the communication. For example, according to the conventional technique by which a licensed band is used outdoor, while an unlicensed band is used indoor, the procedure of having the communication terminal recognize the frequency of the unlicensed band is not taken into consideration. For this reason, it is difficult to perform communication while ensuring that the unlicensed band is used.
Further, also according to the conventional technique by which the CA is implemented by using a licensed band and an unlicensed band, the procedure of having the communication terminal recognize the frequency of the unlicensed band is not taken into consideration. For this reason, also with this conventional technique, it is difficult to perform communication while ensuring that the unlicensed band is used.
Further, also according to the conventional technique by which the CA is implemented between a licensed band and an unlicensed band while using group IDs so as to perform a synchronization process by using the licensed band, the procedure of having the communication terminal recognize the frequency of the unlicensed band is not taken into consideration. For this reason, also with this conventional technique, it is difficult to perform communication while ensuring that the unlicensed band is used.
In view of the circumstances described above, it is an object of the present disclosure to provide a radio communication system, a base station, a communication terminal, and a radio communication system controlling method capable of performing communication while ensuring that an unlicensed band is used.